Atomization is a process in which a liquid composition is broken up into a mist of fine liquid droplets. Atomization is involved in a wide range of industrial applications, including humidification processes, coating operations in which the atomized liquid composition is caused to form a coating on a substrate, vaporization processes, materials transport processes, inhalation delivery processes, and the like.
Plain-jet, air blast atomization is an atomization technique in which a relatively high velocity gas stream is caused to collide with a stream of the liquid composition to be atomized. In a typical plain-jet, air blast atomization operation, streams of the gas and liquid composition are supplied to separate passageways of a plain-jet, air blast device, typically in the form of a nozzle. The gas stream is then shaped and discharged through an annularly shaped orifice of the apparatus as a converging, annularly shaped, high velocity stream. The liquid stream is discharged from an orifice located in approximately the center of the annularly-shaped gas orifice such that the discharged liquid stream is surrounded by the converging annulus of gas. Atomization results when the discharged gas stream convergingly collides with the discharged liquid stream in front of the apparatus.
Conventional plain-jet, air blast atomization devices tend to have a number of drawbacks. First, these devices tend to discharge the gas in a high frequency, pulsed fashion due to sonic vibrations that tend to develop in the gas stream. The energy of the gas/liquid collision thus varies with the frequency of the gas pulses. As a consequence, the atomized liquid droplets will have a size distribution that cyclically varies in accordance with the pulses as well. This size variation is a drawback in many operations, including coating operations in which the size variation of the droplets could result in nonuniform coating thicknesses. It would be desirable, therefore, to be able to generate a smooth, continuous, pulseless flow of gas so that the energy of collision, and hence the size and number density of the atomized droplets, would be more uniform.
Some of the currently known plain-jet, air blast devices also are not well-suited for handling sticky and/or relatively viscous liquids. These kinds of materials can plug or otherwise be difficult to convey in such devices. Yet, there are many applications, such as applying smooth coatings of adhesives onto a substrate, in which it would be desirable to be able to atomize such liquids in a smooth, continuous, reliable manner.
Slippage is another problem that affects plain-jet air blast devices. Slippage results because the gas/liquid collision does not break up the liquid composition into the final atomized state in the first instance. Instead, collision initially breaks the liquid into threads and ligaments that stretch and slenderize as the liquid is driven by the gas away from the apparatus. At some point, the stretched, slenderized bodies of liquid collapse and form the fully atomized liquid droplets. Thus, there is some time delay between the initial time of collision and the time that the final atomized state is reached. Accordingly, it would be desirable to carry out plain-jet, air blast atomization in a manner that minimizes slippage. For a discussion of slippage and principles of atomization in general, see, e.g., Lefebvre, A. H., Atomization and Sprays, Hemisphere Publishing Corp., U.S.A. (1989); and Harari et al., Atomization and Sprays, vol. 7, pp. 97-113 (1997).